Optical device

ABSTRACT

An optical device for use as an optical switch, optical amplifier or optical logic circuit comprises an optical waveguide such as a W profile optical fiber formed from an assembly of optically conductive media having different refractive indices, at least one of which is non-linear, whereby an optical signal with a single mode is guided or not guided along the waveguide in accordance with the intensity of the signal; and coupling means for coupling at least two optical signals into an input end of the waveguide. A Y-coupler is coupled with the optical waveguide to couple at least two optical signals into an input end of the waveguide. One of the optical signals may comprise a bias signal.

FIELD OF THE INVENTION

The invention relates to an optical device for example for use as anoptical amplifier or optical switch.

BACKGROUND AND SUMMARY OF THE INVENTION

There is an increasing requirement in the field of opticalcommunications and optical logic circuitry to develop simple opticalcomponents to achieve functions such as switching, logic operations,amplification and the like.

In accordance with one aspect of the present invention, an opticaldevice comprises an optical waveguide formed from an assembly ofoptically conductive media having different refractive indices, at leastone of which is non-linear, as hereinafter defined whereby an opticalsignal with a single mode is guided or not guided along the waveguide inaccordance with the intensity of the signal; and coupling means forcoupling at least two optical signals into an input end of thewaveguide.

In accordance with a second aspect of the present invention, a method ofoperating such an optical device comprises injecting a first opticalsignal into the waveguide, the intensity of the first signal being suchthat the signal is substantially cut-off; and injecting a second opticalsignal into the waveguide with an intensity such that the resultant ofthe first and second signals has an intensity sufficient to be guided bythe waveguide.

This invention use of the properties of inhomogeneous media having anon-linear refractive index such that the waveguide switches from aguiding to a non-guiding condition depending upon the intensity of thetotal incident light. This self-guiding property can be used in a numberof different applications.

Preferably, the waveguide comprises at least three nested media, the twoinner media having refractive indices in the guiding conditionrespectively greater and less than the refractive index of the outermedium, the refractive index of at least one of the inner media beingnon-linear.

Typically, the waveguide comprises a core, an inner cladding surroundingthe core, and an outer cladding surrounding the inner cladding.

For example, a typical optical fibre has a core with a refractive indexhigher than that of the outer cladding, and an inner cladding with arefractive index lower than the outer cladding.

An example of a suitable optical fibre is a fibre having a W profilewhich has a fundamental mode cut-off which can be moved to the desiredwavelength by suitable design. Silica has a self-focussing Kerrcoefficient which would require powers of about 100 kW. Other materialswhich could be used include doped silica and silica W capilliariescontaining highly non-linear organic materials.

By "non-linear" we mean that the refractive index of the medium (n₁)varies with intensity. Typically, this will be according to the formula:

    n.sub.1 =n.sub.0 +n.sub.2 |E|.sup.2

where n₀ is the linear refractive index of the medium, n₂ is the Kerrcoefficient, and |E|² is the intensity of the incident light.

Typically, |E|² n₂ is very much less n₀ and thus the non-linear effectis only apparent at high intensities. The invention is concerned withwaveguides where the intensity of the incident light and the Kerrcoefficient are such that n₀ is preferably not more than two to threeorders of magnitude greater than n₂ |E|².

It should be understood that the refractive index n₂ may increase ordecrease with intensity.

In a homogeneous medium the non-linear refractive index of the waveguidewill cause a transverse optical field to disperse at low intensities butto collapse to a self-focussing singularity at high intensities. Inother media, a self-defocussing non-linearity will occur where therefractive index decreases with intensity. The type of non-linearitydepends upon the sign of the Kerr coefficient. In an inhomogeneousmedium it is possible for guiding to occur before catastrophic selffocussing occurs.

The device is operated in the following way for a waveguide with aself-focussing non-linearity; if a waveguide with a self-defocussingnon-linearity is used then the words in brackets apply. At a frequencynear to the fundamental mode cut-off the optical field is not wellguided. At low (high) powers most of the power launched diffracts outinto the cladding. As the intensity is increased the refractive indexderived from the non-linearity causes the cut-off to move to lower(higher) frequencies. This means that the optical field at the operatingfrequency is more (less) guided than before and hence that less (more)power diffracts into the cladding. If the power confined in some centralregion is defined as the output power then we have a device whose outputpower is a non-linear function of the input power.

In other words the guiding/non-guiding property depends on the resultantrefractive index experienced by the optical field.

Preferably all the media are non-linear.

Preferably, the device further comprises separation means associatedwith an output end of the waveguide whereby portions of optical signalsleaving a core region of the output end are separated from otherportions of the signals. By providing separation means, the variation ofoutput power with input power just described can be utilised. Typically,the separation means may comprise a monomode optical fibre which, in thecase where the optical waveguide comprises an optical fibre, is splicedto the optical waveguide. In this way, only that portion of the opticalsignal passing through the core of the optical waveguide is coupled intothe monomode optical fibre as the output signal.

Conveniently, the device further comprises bias signal generating means,such as a laser, for injecting optical bias signal into the input end ofthe waveguide, the intensity of the bias signal being such that thewaveguide operates in its non-linear region and the bias signal iscut-off. This enables the device to be used in a variety ofapplications. For example as a switch, the device, when biassed into itscut-off region, can be caused to switch on or off by applying a suitableswitching signal in addition to the bias signal causing opticalradiation to be guided or not respectively. The bias and the signal mustbe phase aligned.

The device can also be used as the basis of a logic element such as anAND gate in which a number of inputs (two or more) are applied to aninput end of the optical waveguide and optical radiation is onlyreceived at an output end of the waveguide if the total incidentintensity is sufficient to cause self-guiding to occur. It will beappreciated that in these applications the optical waveguide must have asuitable length. A device with a self-defocussing non-linearity could beused as the basis of a NAND gate.

Conveniently, the device further comprises coupling means such as a Ycoupler, one input arm of the coupler being coupled with the bias signalgenerating means, the other input arm of the coupler being coupled withan input signal, and the output arm of the coupler being coupled withthe waveguide.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of an optical device and a method of operating the assemblyaccording to the present invention will now be described with referenceto the accompanying drawings, in which:

FIG. 1 illustrates the variation in refractive index across the diameterof an optical fibre having a W profile;

FIG. 2 illustrates the variation of effective refractive index with thetransverse wavelength of incident optical radiation;

FIGS. 3a-3c illustrate the variation in intensity of a transmittedoptical signal with three different initial intensities respectively;

FIG. 4 illustrates graphically the way in which output power of anoutput optical signal varies with input power for three differentoptical fibre lengths; and;

FIG. 5 illustrates schematically an example of the assembly.

DETAILED DESCRIPTION

A typical W profile optical fibre has an outer cladding, a doped silicacore with a refractive index higher than that of the outer cladding andan inner cladding with a refractive index lower than the outer cladding.An example of the variation of refractive index across a typical Wprofile optical fibre is shown by a line 1 in FIG. 1. Consider theexample of two transverse optical fields having different wavelengths.Different proportions of these optical fields will travel through thecentral core region 4 and the inner cladding region 5 and the outercladding region 2 of the optical fibre, so that the effective (oraverage) refractive index experienced by each field will be different.Thus, the effective refractive index (n_(eff)) experienced by one fieldwill be as shown by the dashed line 6 in FIG. 1 while the effectiverefractive index experienced by another field will be as indicated bythe dashed line 7. Thus, the effective index is the average index seenby the field which approaches the outer cladding index as the wavelengthis increased.

FIG. 2 illustrates the overall variation of effective refractive indexwith wavelength as indicated by a line 8. For comparison, a line 9indicates the same variation but for an optical fibre which does notexhibit a fundamental mode cut-off. It will be seen in FIG. 2 that thereis a particular wavelength λ_(c) at which the effective refractive indexis equal to the outer cladding index at which point propagation throughthe fibre ceases.

It has already been mentioned above that the optical fibre chosen has anon-linear refractive index. This means that the form of the profileshown in FIG. 1 can be varied by varying the intensity of the incidentoptical signal. The result of this is that the curve 8 shown in FIG. 2can be shifted by, in the case of a self-focussing fibre, increasing theintensity of the injected optical radiation so that a new curve 10 isdeveloped having a higher cut-off wavelength λ'_(c). Thus, if opticalradiation having a wavelength λ_(c) is initially injected into theoptical fibre at an intensity corresponding to that followed by the line8 in FIG. 2 no transmission will occur. However, a small increase inthat intensity will modify the refractive index and hence the effectiverefractive index of the optical fibre from its previous outer claddingvalue to a value (n') greater than the outer cladding value allowing thesignal to be guided along the fibre. It is on this principle that theinvention is based.

We have performed a theoretical analysis on propagation along a Wprofile fibre. To do this, we solved the scalar wave equation

    ∇.sup.2 E(xyz)+(k.sup.2 n.sup.2 (r)-β.sup.2)=0

    k=2π/λ

for the continuous wave (c-w) electric field, E(z,x,y), where z is thepropagation distance along the fibre, x and y the two transversedirections and λ the wavelength, using the beam propagation method forthe forward travelling wave. The refractive index n(r,|E|²) is given by

    n(r,|E|.sup.2)=n.sub.0 +n.sub.2 ·|E(r)|.sup.2,

where n₀ (r) is the fibre refractive index profile and n₂ is the Kerrcoefficient.

In these calculations we imposed rectangular symmetry in order to usefast fourier cosine transforms to improve calculating efficiency.Experiments were made on the number of Fourier modes needed and on theintegration step; the values chosen were adequate to give severalfigures accuracy for the calculations described here. We also checkedthat sufficient points were included to accurately represent bound modesof cylindrical symmetry. The calculations were carried out over adistance of 30 mm, using (192)² Fourier modes in a periodic box of sides300 microns, and an integration step of 1.5 microns. At the input to thefibre we launched a beam profile given by the bound mode of a W-fibrewith a similar shape, but a sufficiently high core refractive index tobind the mode.

FIG. 3a shows how the beam intensity profile across the fibre diameterslowly diffracts out of the core with propagation down the fibre, forthe linear case where n₂ =0; FIGS. 3b and 3c show the profiles forhigher launched power. As we have neglected loss in this problem, thetotal energy must be conserved. However, the amount within a finiteradius from the centre reduces, as energy is diffracted into thecladding. The energy confined within a 10 micron radius decreases muchmore slowly than for a fibre with no structure in the refractive index(the beam would disperse in a distance of about a millimeter in ahomogeneous fibre).

We then studied the effect of a nonlinear refractive index on thispropagation. FIG. 4 shows the fraction of the launched energy stillconfined within a 10 micron radius for fibres 7.5 mm, 15 mm and 30 mmlong as a function of n₂ ·|E(0)|². We can see from this figure that thefibre can be used as a threshold device. If the centre part of the beamwere coupled into (say) a second standard momomode fibre then thefraction of the total energy launched into the propagating mode woulddepend on the input power.

An example of an assembly incorporating a W-profile fibre is shown inFIG. 5. In this example, a W profile optical fibre 11 is connected to aY coupler 12. One of the input arms 13 of the Y coupler 12 is coupledwith a laser 14 while the output arm of the coupler 12 is connected tothe fibre 11. A momomode optical fibre 15 is spliced to the central coreof the fibre 11.

When the assembly shown in FIG. 5 is to be used as an optical switch oramplifier, a bias optical signal is supplied from the laser 14 to theoptical fibre 11. The intensity of the bias signal is selected to lie onthe portion of the transmission power profile (FIG. 4) close to the steppart of the profile. For example, for a 30 mm optical fibre, the biasintensity will be chosen to lie at about the position 16. By biassingthe fibre at this position, a small control signal applied along theother input arm 17 of the Y coupler 12 will cause the total incidentintensity to lie at a position towards the top of the graph shown inFIG. 4 resulting in a relatively high output power.

Typically, the transverse form of the bias signal alone will be similarto that of FIG. 3a. Thus, at the output end of the optical fibre 11 theenergy is spread over a large area with a very small proportion withinthe central core region so that only a very small amount of the signalis coupled into the optical fibre 15. When a signal is supplied alongthe arm 17, this adds to the intensity of the bias signal to such anextent that the total intensity changes to, for example, the position 18in FIG. 4. This causes the optical fibre 11 to switch into itsself-guiding mode in which a large proportion of the initial totalintensity is guided within the core to the output end of the fibre asshown in FIG. 3c. Thus, a large intensity signal is coupled into theoptical fibre 15.

In another arrangement (not shown) the optical fibre 11 could be used asan AND circuit with two input signals being fed along the arms 13, 17(laser 14 being omitted) so that only when signals of sufficientintensity are fed along both the arms 13, 17 is there sufficientintensity from the resultant signal for self-guiding to produce a largeoutput power within the central core of the optical fibre 11.

I claim:
 1. An optical device comprising:an optical waveguide responsiveto the intensity of light incident thereon to guide or not guide anoptical signal from an input end to an output end of said waveguide,said waveguide having a core region, an outer cladding region and atleast one further region disposed therebetween which has a lowerrefractive index than said outer cladding region, at least one of saidregions having a refractive index which varies with respect to theintensity of said incident light; coupling means including first andsecond input means for coupling at least two optical signals into saidinput end of said waveguide; and separation means for separatingportions of optical signals leaving said core region at said output endof said waveguide from other portions of the signals travelling in saidouter cladding region and said at least one further region.
 2. A deviceaccording to claim 1, wherein the separation means comprises a monomodeoptical fibre.
 3. A device according to any one of claims 1 or 2,further comprising bias signal generating means, connected, in use, tosaid coupling means, for injecting an optical bias signal into the inputend of the waveguide, the intensity of the bias signal being such thatthe waveguide operates in its non-linear region and the bias signal iscut-off.
 4. A device according to claim 3, wherein said coupling meansoperates to couple an input signal with the waveguide.
 5. A deviceaccording to claim 4, wherein said coupling means comprises a Y coupler,and said first input means comprises one input arm of the couplercoupled with the bias signal generating means, and said second inputmeans comprises the other input arm of the coupler coupled with saidinput signal, and the output arm of the coupler being coupled with thewaveguide.
 6. A device according to claim 1, wherein at least one of theregions disposed within said outer cladding has a refractive index whichvaries with respect to the intensity of said incident light.
 7. A deviceaccording to claim 6, wherein all of the regions have a refractive indexwhich varies with respect to the intensity of said incident light.
 8. Adevice according to claim 7, wherein the waveguide comprises an opticalfibre.
 9. A device according to claim 8, wherein said fibre has a Wrefractive index profile.
 10. A method of operating an optical deviceincluding an optical waveguide responsive to the intensity of lightincident thereon to guide or not guide an optical signal from an inputend to an output end of said waveguide, said waveguide having a coreregion, an outer cladding region and at least one further regiondisposed therebetween which has a lower refractive index than said outercladding region, at least one of said regions having a refractive indexwhich varies with respect to the intensity of said incident light;coupling means for coupling at least two optical signals into said inputend of said waveguide; and separation means for separating portions ofoptical signals leaving said core region at said output end of saidwaveguide from other portions of the signals travelling in said outercladding region and said at least one further region, said methodcomprising the steps of:injecting a first optical signal into saidwaveguide via said coupling means, said first signal having an intensitysuch that the first signal would be substantially cut-off by saidwaveguide and would not in isolation be guided by the waveguide; andinjecting a second optical signal into the waveguide via said couplingmeans, said second signal having an intensity which is sufficient tocause said first and said second signals to be guided by the waveguide.11. A method according to claim 10, including the step of separatingportions of optical signals leaving said core using said separationmeans.
 12. A method according to claim 11, wherein said separating stepis performed using a monomode optical fibre as the separation means. 13.A method according to claim 10, wherein at least one of the regionsdisposed within said outer cladding has a refractive index which varieswith respect to the intensity of said incident light.
 14. A methodaccording to claim 10, wherein all of the regions have a refractiveindex which varies with respect to the intensity of said incident light.15. A method according to claim 14, wherein the waveguide comprises anoptical fibre.
 16. A method according to claim 15, wherein said fibrehas a W refractive index profile.
 17. A method according to claim 10,wherein said step of injecting a first optical signal includes the stepof injecting an optical bias signal into the input end of the waveguide,the intensity of the bias signal being such that the waveguide operatesin its non-linear region and the bias signal is cut-off.
 18. A methodaccording to claim 17, further including the step of coupling an inputsignal with the waveguide via said coupling means.
 19. A methodaccording to claim 18, wherein said coupling means comprises a Ycoupler, having two input arms and an output arm, further including thesteps of coupling said bias signal to one input arm of the coupler,coupling the input signal to the other input arm of the coupler, andcoupling the output arm of the coupler to the waveguide.